Chapter 13 — Viruses, Viroids, and Prions

What are viruses?

Minuscule, acellular infectious agents containing either DNA or RNA that cannot reproduce independently.

Why can't viruses reproduce independently?

They lack metabolic pathways and use the host cell's machinery to make more viruses.

What is a virion?

The extracellular form of a virus, consisting of nucleic acid surrounded by a protein capsid; some also have an envelope.

What is the intracellular state of a virus?

The capsid is removed, leaving the viral nucleic acid inside the host cell.

What genetic material can viruses contain?

DNA or RNA, but never both; it may be single-stranded or double-stranded.

What is a capsid?

A protein coat that protects viral nucleic acid and helps the virus attach to host cells.

What are the three basic viral shapes?

Helical, polyhedral, and complex.

What is a viral envelope made of, and where does it come from?

A phospholipid bilayer and proteins acquired from the host cell's membrane during viral replication or release; it may contain viral glycoprotein spikes.

What are the five stages of the lytic cycle?

Attachment, entry, synthesis, assembly, and release.

What usually happens to the host cell during the lytic cycle?

The host cell dies and undergoes lysis as new viruses are released.

What is lysogenic replication?

A modified viral replication cycle in which infected host cells reproduce normally for generations before they eventually lyse.

What are the three ways animal viruses enter host cells?

Direct penetration, membrane fusion, and endocytosis.

What is the difference between positive-sense and negative-sense RNA?

Positive-sense RNA can act directly as mRNA; negative-sense RNA cannot be directly translated.

What is the first step in the HIV life cycle?

Binding or attachment: HIV attaches to receptors on the surface of a CD4 cell.

What happens during HIV fusion?

The HIV envelope fuses with the CD4 cell membrane, allowing the virus to enter the cell.

What is reverse transcription in HIV?

The enzyme reverse transcriptase converts HIV RNA into DNA.

What happens during HIV integration?

The enzyme integrase inserts viral DNA into the DNA of the CD4 cell.

What happens during HIV replication and assembly?

The host cell machinery makes HIV proteins; viral RNA and proteins assemble into an immature, noninfectious virus.

What happens during HIV budding and maturation?

New immature viruses leave the CD4 cell; protease cuts long protein chains so the virus becomes mature and infectious.

How do HIV medications interfere with the viral life cycle?

Different inhibitors target attachment, fusion, reverse transcription, integration, or protease activity.

What is neoplasia, and what is metastasis?

Neoplasia is uncontrolled cell division; metastasis is the spread of tumors to other locations.

What are protooncogenes and tumor suppressor genes?

Protooncogenes promote cell growth and division; tumor suppressor genes slow division, repair DNA, and can signal damaged cells to undergo apoptosis.

How are viruses involved in cancer?

Viruses can carry oncogenes, activate host oncogenes, or interfere with tumor suppression.

How are viruses cultured in a laboratory?

They must be grown inside host cells, using mature organisms, embryonated eggs, or cell cultures.

What are advantages and disadvantages of using mature organisms to culture viruses?

Phages can be grown in bacteria and counted using plaques; laboratory animals can be expensive and difficult to maintain, and raise ethical concerns.

What are advantages of embryonated chicken eggs for culturing viruses?

They are inexpensive, have large cells, are free of contaminating microbes, contain nourishing yolk, and provide embryonic tissues suitable for viral growth.

What is a cell culture?

Isolated cells from an organism grown in a medium or broth.

What are viroids?

Very small, circular, infectious pieces of single-stranded RNA that cause plant disease; they lack a capsid and do not code for proteins.

What are prions?

Proteinaceous infectious agents that cause disease by changing the shape of normal cellular PrP proteins.

What is the difference between normal PrP and prion PrP?

Normal cellular PrP has alpha-helices; disease-causing prion PrP has beta-pleated sheets.

How does prion conversion occur?

Disease-causing prion PrP causes normal cellular PrP to refold into the disease-causing form.

What can inactivate prions according to the lecture slides?

Incineration or autoclaving in concentrated sodium hydroxide; standard sterilization procedures do not deactivate them.